1. Field of the Invention
This invention relates to variable width, constant impedance baffles for maintaining proper air flow distribution throughout a partially populated electronic circuit mounting frame.
2. Description of the Prior Art
It is a common practice to mount electronic circuitry on planar printed circuit boards to form printed circuit board assemblies. The printed circuit board assemblies are then assembled in a closely spaced parallel relationship within a frame or cabinet which is often referred to as a chassis. The frame is provided with a plurality of edge slot sets, each receiving and mounting a printed circuit board assembly. Typically printed circuit board assemblies are slideably inserted into the slot sets from a front side of the frame and have printed plug connectors for engaging and providing electrical connection to a receptacle connector at the opposite, back side of the frame. Usually the receptacle connectors are part of a backplane wiring assembly which provides a desired interconnection between printed circuit board assemblies.
It is often necessary to provide forced air cooling of electronic components of the printed circuit board assemblies and for this purpose a fan is disposed to blow air through the frame in a direction parallel to the planes of the printed circuit board assemblies. Frequently two or more mounting frames are stacked so that a single fan can blow air through a plurality of mounting frames.
Ordinarily the printed circuit board assemblies will be populated with similar components such as dual-in-line packages and are uniformly spaced so that they present approximately equal impedances to the flow of cooling air and cause the air to be uniformly distributed throughout the frame. However, a problem arises when a frame is less than fully populated with printed circuit board assemblies. For example, a memory frame might be sold in a minimum size configuration with empty slots available for expansion or a computer mounting frame might have slots available for the addition of more memory or additional input/output boards.
Whenever one or more vacant slots occur in a mounting frame the distribution of impedance becomes unequal and cooling air preferentially flows through the vacant slots. As a result, the cooling air flow to the mounted printed circuit board assemblies is reduced and their circuit components tend to operate at high temperatures. This results in decreased reliability or undesirable design tradeoffs such as selection of high temperature components, decreasing of component density on a circuit board or greater use of expensive heat sinks. Furthermore, when an uneven impedance distribution causes an uneven air flow in one frame, the air flow in adjacent frames will also be uneven, whether or not the adjacent frames are fully populated.
A number of schemes have been employed to evenly distribute the air flow in a partially populated frame. One scheme employs "unit" baffles or dummy printed circuit board assemblies approximating the air flow impedance of an operative printed circuit board assembly. This scheme can require the utilization of a large number of "unit" baffles. For example, 20 open slots in a frame would require the use of 20 "unit" baffles.
A special population distribution is also used on occasion. That is, the printed circuit board assemblies are distributed uniformly across the available slots in a chassis. However, as the number of boards changes the air flow and cooling changes. Furthermore, special design considerations may be required to permit an even distribution of circuit boards to be maintained as the number of printed circuit board assemblies in a frame is changed.
Rigid baffles providing an air blockage have also been used. They are suitable only for a given size of vacant space within a chassis. Different baffles must be assembled for each different number of vacant slots, thus increasing inventory as well as manufacturing problems. Because air flow is blocked in the vacant regions of a mounting frame, air flow distribution changes with the number of vacant slots in the frame and uneven air flow is provided to adjacent frames.